The Universal Mobile Telecommunication System (UMTS) is one of the third generation mobile communication technologies designed to succeed GSM. 3GPP Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to improve the UMTS standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, and lowered costs. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. In an UTRAN and an E-UTRAN, a user equipment (UE) is wirelessly connected to a radio Base Station (BS) commonly referred to as a NodeB and an evolved NodeB (eNodeB) respectively. Each BS serves one or more areas referred to as cells.
In E-UTRAN, Orthogonal Frequency Division Multiple Access (OFDMA) technology is used in the downlink and single carrier frequency division multiple access (SC-FDMA) in the uplink. In both uplink and downlink the data transmission is split into several sub-streams, where each sub-stream is modulated on a separate sub-carrier. Hence in OFDMA based systems, the available bandwidth is sub-divided into several resource blocks (RB). A resource block is defined in both time and frequency. According to the current assumptions in the 3GPP standard, a resource block size is 180 KHz and 0.5 ms in the frequency and time domains respectively. The overall uplink and downlink transmission bandwidth may be as large as 20 MHz per frequency carrier.
According to the 3GPP definition, a heterogeneous network comprises two or more layers, where the layers are served by different types of BSs or BS classes. In a two-layered macro-femto heterogeneous network the macro cell layer and femto cell layer typically comprise macro BS and Home BS (HBS), respectively. FIBS are radio BSs with restricted wireless access, as will be explained later. In co-channel heterogeneous networks, all layers operate on the same carrier frequency.
Hitherto, three LTE BS classes are specified: wide area or macro BS, local area or pico BS, and HBS. Nonetheless additional BS classes such as medium range BS may be introduced in the future. The BS classes differ in that they have different levels of maximum output power and associated minimum coupling loss. Some other requirements such as frequency error and receiver sensitivity may also differ for different BS classes as they are generally optimized for specific deployment scenarios. In LTE, the maximum output power of a local area BS which serves a pico cell, and a HBS which serves a femto cell, is 24 dBm and 20 dBm respectively, when Multiple Input Multiple Output (MIMO) is not applied. For E-UTRAN FDD and E-UTRAN TDD, the HBS maximum output power, Pf, max, antenna, is 17 dBm per antenna port in case of two transmit antennas, or 14 dBm per antenna port in case of four transmit antennas, and so forth. A formula for the HBS maximum output power is given by a general formula according to the following:Pf,max,antenna=20 dBm−10*log10(N)where N is the number of transmit antenna ports at the HBS. Similar scaling is also used for the maximum output power of the pico BS when MIMO is used. The maximum output power of the macro BS is declared by the manufacturer, and may typically be between 43 and 46 dBm. BS power classes or types similar to those specified for LTE are also specified in UTRAN.
A heterogeneous network may enhance capacity in dense traffic areas or hotspots, i.e. small geographical areas with a higher user density and/or higher traffic intensity. Heterogeneous networks may also be used for coverage extension. However, heterogeneous network deployments and in particular the co-channel scenario also bring challenges for which the network has to be prepared to ensure efficient network operation and superior user experience.
The heterogeneous network may constitute of BS employing any one or a mix of technologies such as LTE, High Speed Packet Access (HSPA), GSM, and CDMA2000. There are several frequency bands which have been standardized for multiple technologies, e.g., band 1 at 2 GHz for LTE and HSPA, and band 3 at 1800 MHz for GSM, LTE and HSPA. Hence heterogeneous deployment may even comprise of mixture of technologies. Another example scenario comprises BS that are a mixture of single radio access technology BSs and multi-standard radio (MSR) type BSs.
A HBS, sometimes also called a femto BS, typically serves private premises or small office environments. Another main characteristic of the HBS is that it is typically owned by a private subscriber who has the liberty to install it at any location. The subscriber's operator may also own the HBS, but the location of the HBS may not be fixed. The subscriber may e.g. move the HBS from one part of the house to another. Thus strict network planning may not be possible or may be challenging in case of HBS deployment. This is different from other BS classes which are deployed by an operator according to some well defined principles.
An access control mechanism for the HBS decides if a given UE may or may not connect to that HBS. The selection of the access control mechanism has a large impact on the performance of the overall network, mainly due to its role in the definition of interference. in UTRAN and E-UTRAN, the concept of Closed Subscriber Groups (CSG) exists. According to CSG, only a subset of UEs, defined by the owner of the HBS, may wirelessly access or connect to that particular HBS. Hence wireless access for other UEs is denied by the CSG based HBS.
FIG. 1 illustrates a part of a heterogeneous network mixing macro cells 120 served by macro nodes 110, and cells operating as CSG cells 140 served by HBSs 130. The macro node 110 may be a macro BS, and the HBS 130 may be a home eNodeB in an LTE network. In the illustrated scenario, a macro UE (MUE) 150 which is camped on, connected to, or served by the macro node 110, which is illustrated by the arrow 115, is close to the strong CSG HBS(s) 130. However, the MUE may not be allowed to be served by the CSG cell 140, as the CSG cell 140 has restricted access for certain UEs. The CSG cell 140 may thus be regarded as a non-allowed HBS 130 from this MUE 150 point of view. In this situation, the MUE 150, called the aggressor MUE, may cause significant uplink interference to the HBS 130, which is illustrated by the arrow 155, as the MUE 150 is operating close to the victim HBS 130. The HBS 130 may be referred to as the victim HBS.
Another problem is the downlink interference from the HBS 130 towards the MUE 150. The HBS may be required to lower its maximum output power, i.e. to adjust its downlink power settings, in order to protect the downlink reception quality of the MUE operating in an adjacent carrier frequency. The HBS maximum output power may also need to be adjusted in the co-channel deployment scenario, i.e. when the macro node and the HBS operate on the same carrier frequency. The adjustment of the maximum output power of the HBS to protect the MUEs downlink quality can be ensured by specifying certain requirements for the HBS in the standard. For such a co-channel scenario, i.e. a victim downlink MUE and an aggressor downlink HBS, there are currently no such requirements on HBS. However in near future these requirements are expected to be defined in the standard for LTE HBS. The HBS maximum output power adjustment techniques enable the MUE to operate close to the CSG cells, as the MUE is able to receive downlink signals from its serving macro node with a relatively reduced interference from the HBS.
The downlink maximum output power adjustment, sometimes also called a downlink maximum output power setting algorithm, is thus designed to protect MUEs, and in particular those MUEs that are not allowed to access the CSG cell. The downlink maximum output power adjustment may be used not only for reducing the transmit power, but also to expand the coverage area of a cell such as the CSG cell, or more specifically to restore the maximum HBS coverage when there are no non-CSG MUEs in close vicinity any longer. If no MUEs suffer from high downlink interference from the HBS any longer, the HBS coverage area may be restored to its maximum again.
The side effect with the HBS maximum output power adjustment, making it possible to shrink the HBS coverage and therefore increase the coverage of the macro cells, is that some UEs may change serving cell due to the adjustment. The HBS coverage shrinking results in that some of the UEs belonging to the CSG cell, hereinafter referred to as Home UEs (HUE), may have to be served by the macro node instead. The consequences of this are that:                1. The macro node may have to transmit at a higher power in the downlink to avoid a coverage hole. This may further degrade the downlink reception quality of the HUEs and the MUE.        2. The new cell edge MUEs of the macro cell, which increased its coverage due to the shrinking of CSG cell coverage, are now located close to the HBS and possibly also far away from the new serving macro node. These new cell edge MUEs will therefore likely transmit at a relatively higher power level in the uplink than they would have done as HUEs in the CSG cell if no CSG coverage shrinking was performed. The uplink interference experienced at the HBS is therefore increased, thus deteriorating the uplink reception quality. This will in particular impact the uplink reception quality of the HUEs which may already be experiencing high interference from the non-CSG MUEs.        